Discrimination against fat people might not be all the fault of bullies, international scientists say.
The sight of an obese person could trigger feelings of disgust
They found, for some people, the sight of obesity sparked strong subconscious reactions, such as disgust.
The Evolution and Human Behaviour study suggests this is part of a deep-seated behavioural reponse designed to help detect and avoid those with infections.
But experts said there was no excuse for pre-meditated prejudice against obese people.
The researchers looked at the results of a questionnaire completed by more than 250 people from the University of British Columbia.
They found that, just as rotten food can trigger nausea, the sight of someone obese could trigger reactions such as disgust.
And the reactions were stronger in people who were most afraid of disease.
People who agreed with the statement, "It really bothers me when people sneeze without covering their mouths" were more likely to agree with, "If I were an employer looking to hire, I might avoid hiring a fat person."
The researchers say because obesity represents a deviation from what we regard as a 'normal' body morphology, and so triggers the response because it could be the sign of a disease caused by pathogens, such as bacteria or viruses.
This is similar to the response people might feel if they saw someone with a rash, and the authors say people can be highly sensitive to such visual triggers even if they know a person does not actually have a disease.
However, although these negative responses to certain triggers could aid survival by helping people detect and so avoid those with infections, it could also cause negative attitudes towards obese people.
The researchers said their results indicated that prejudice against obese people might not be rooted in psychological mechanisms, but in this innate bahavioural response.
They said: "This is sobering, but encouraging because it provides clues toward the possible reduction of this for of prejudice.
"Our results suggest that it might be ameliorated through interventions that focus on individual's often irrational concerns about infectious disease."
Dr Ian Campbell, medical director of charity Weight Concern, said children as young as seven had been shown to have an in-built discrimination against obese people, so he was "not surprised" that such a prejudice could have an evolutionary basis.
He said: "Being very obese is associated with a greater risk of ill health, so I'm not surprised by this."
But he added that in recent years social factors had also played an important role in making obese people the butt of jokes.
And Dr Colin Waine, chairman of the National Obesity Forum, said prejudice against obese people existed even amongst healthcare professionals, and that it must be removed.
He said: "Human beings have been given powers of reason, so people can overcome the discrimination.
"Attitudes to coloured people have become healthier, even though once they were looked down upon.
"Obese people are currently blamed for their condition, but people should try to overcome that attitude."